Heretofore, internal combustion engines of the reciprocating type have been constructed of metals in forms best suited for their fabrication in such materials. However, due to these materials prior art engines require supplemental cooling and lubrication in order to function properly with adequate durability. These cooling and lubrication requirements further require provisions for fluid circulation and heat rejection accessories that can be burdensome in many applications. Aircraft applications of such engines are particularly sensitive to the installation of such accessories because of the weight and aerodynamic drag associated with their proper usage. In addition, the control of fluids in aircraft engines and their remote accessories such as radiators, oil coolers, pumps, oil sumps and the like is complicated because a fixed gravitational orientation can not be relied upon to disengage vapors and liquids and establish fluid levels.
A further disadvantage of most prior art engine constructions for aircraft applications is their dependence on increased output shaft speed as a means of reducing weight per unit of power output. Because propellers function efficiently only with limited rotational speeds, most light-weight engines of the prior art type require speed-reducing gear boxes, and perhaps even variable ratio transmissions, to properly match their outputs to suitable propellers. Such mechanical accessories have cooling and lubrication requirements of their own and can add significant weight, .cost and complexity to the installation, particularly for small-engine and high-altitude applications. Such speed constraints are not limited to aircraft applications. Certain alternators and compressors represent other important drive applications that are so limited.
Most prior art engines employ structural arrangements, assemblies and mechanisms that are highly dependent on the tensile properties of the customary metallic materials which have limited temperature tolerance, expand significantly when heated and are prone to galling under sliding and rubbing contact. They require sophisticated cooling and lubrication schemes to maintain their mechanical and structural integrity and their weight and balance is highly sensitive to increases in cylinder working pressures and rotational speeds. Thus, prior art engines that operate on the diesel cycle are somewhat heavier and larger than their spark ignition counterparts and they also present greater lubrication, cooling and balancing burdens. This accounts, to a large extent, for the lack of acceptance, heretofore, of prior art type diesel engines for aircraft applications notwithstanding their potentially superior flight-worthiness, safety, fuel economy and fuel flexibility characteristics.
Various attempts have heretofore been made to overcome some of these problems by designing diesel engines with large heat retention capacities. Examples of such "adiabatic engine" are those manufactured by Adiabatic Inc. and Cummins. These adiabatic engines utilize insulated parts, heat tolerant components and high-temperature tribology or friction controls. However, such friction controls require advanced chemistry for liquid lubrication. What is needed is an adiabatic engine that overcomes these shortcomings.
With rare exceptions, prior art reciprocating engines, adiabatic or otherwise, utilize crankshafts and connecting rods for the translation of reciprocating to rotary motion. This arrangement has been successfully applied to engines comprised of from one to many cylinders laid out in various configurations such as in a single line of cylinders parallel to the crankshaft, banks of inline cylinders disposed around the crankshaft, radial cylinder dispositions and opposed-piston arrangements using one or more crankshafts geared together. A few crankshaft-type engines are known which have been constructed with parallel cylinders axially aligned in a barrel arrangement around the crankshaft or with inline cylinders transverse to the crankshaft. Both of these types rely on additional auxiliary mechanisms such as gear trains, rocker arms, wobble plates, universal ball joints and the like for the translation of power.
Prior art engines that utilize crankshafts provide no mechanical advantage in the conversion of piston motion to shaft torque. Furthermore, eccentricities in connecting rods and the like produce side loads in the reciprocating pistons which give rise to friction and vibration. Another disadvantage of crankshaft-type engines is the complex load path that must be structurally accommodated in maintaining the mechanical integrity of the engine. Typically, such loads are passed through the cylinder walls which must also handle the stresses due to combustion. As a result, the cylinders must be constructed of materials having high tensile strengths. Due to the complex forms of the structures required, metallic materials constitute the only economic and durable means of construction, and then only if an abundance of cooling and lubrication is used. Furthermore, crankshafts, by nature, must span the length of the engine. Because of this, as well as a poor structural geometry for the loads imposed, crankshaft engines require somewhat more weight, strength and stiffness in the shaft, bearings and supporting structure to obtain an adequate degree of torsional rigidity and structural integrity.
The axial piston or barrel configuration typified by the prior art engines of Herrmann, Sterling/Michel and others offers improved compactness, structural efficiency and frontal area. These characteristics are desirable for an engine. However, none of these characteristics has been obtained in the prior art with the use of thermally tolerant and self-lubricated materials in the principal parts. All of these prior art engines rely on the established principles of ironmongery, which succeeds only with proper cooling and lubrication. None of the prior art engines suggests the use of non-metallic construction or arrangements, hence, the burdens of supplemental cooling and lubrication remains.
Many of these prior art engines, such as Junkers, Hill and Sterling/Michel, have utilized opposed-piston arrangements which avoid the use of cylinder heads and the stresses, dynamic forces, seals, attachments and fastenings attendant thereto. Although this arrangement is limited to two-stroke-cycle operation, this can be advantageous for some applications, provided aspiration and cylinder scavenging can be properly attended. Other advantages of the opposed-piston arrangement include reduced combustion chamber heat losses, improved compactness for a given cylinder displacement and reduced piston speed for a given power output.
For example, the Sterling/Michel engine includes an opposed piston arrangement that utilizes a double swashplate for translating axial to reciprocating motion (see, Heldt, P. M., High Speed Diesel Engines, 4th Ed., Nyack, N.Y., 1943, pp. 308-309). However, the Sterling/Michel engine has swashplate followers which impart significant side loads. Furthermore, the engine requires a separate scavenging system and supplemental lubrication. Finally, the Sterling/Michel swashplates are single harmonic, thereby yielding only one power stroke per revolution.
The Junkers engine utilizes two crankshafts in an inline cylinder, opposed piston configuration, thus also yielding only one power stroke per revolution (see Heldt, pp. 320-326). Furthermore, the articulated piston/crankshaft arrangement imparts significant side loads as well. The Junkers engine also utilizes a separate scavenging system, requiring appurtenances which add to the complexity and weight of the engine structure.
The Hill engine has opposed pistons with a single crankshaft/rocker arm assembly that is transverse to the center of the cylinder (see Heldt, p. 310). Thus, it too has side load problems.
Sterling/Michel, Junkers and Hill all used opposed pistons, but none foresaw the opportunity for constructing their engines in a manner that could utilize in any significant respect thermally tolerant and self-lubricated materials. Further, all utilize reciprocating-to-rotary conversion mechanisms that impart side loads on their pistons and which cannot provide any mechanical advantage in the production of torque other than by the familiar method of increasing the piston stroke and/or combustion pressure. Finally, none of these prior art engines included integral aspiration and scavenging means, thus necessitating external or add-on appurtenances such as additional scavenge pump cylinders or separate mechanically-driven blowers.
There is a recently disclosed (date unknown), two-stroke-cycle, opposed piston engine which has significantly reduced or eliminated side loads on the pistons (see the DARPA/Land System Office engine in the Advanced Research Projects Agency Brochure, page 38). This engine utilizes four crankshafts, two counter-rotating crankshafts on each cylinder end. Due to the counter-rotating crankshafts, each having opposing connecting rods attached to a piston, the net side load on each piston is approximately zero. However, this engine structure is mechanically very complicated and does not lend itself to the use of thermally tolerant materials.
Another prior art engine, that of Herrmann (U.S. Pat. Nos. 2,243,817, 818, 819, and 820, all issued in 1941) teaches the use of a double harmonic barrel cam engine. The Herrmann engine utilizes a single cam arrangement in a four-stroke cycle axial cylinder configuration having improved torque multiplication, reduced piston side loads and lower torsional vibrations in the output shaft. However, Herrmann did not anticipate or suggest the use of double-harmonic cams in an opposed piston engine having an axial cylinder arrangement. Furthermore, Herrmann's engine operates on a four-stroke-cycle. Thus, even though Herrmann's double harmonic cam increases the number of piston strokes per shaft revolution, it only obtains one power stroke per revolution. Any further increase in torque output would require the use of a two-stroke-cycle engine. Such an attempt to utilize the Herrmann single cam teachings in a two-stroke-cycle engine would be encumbered by the need for highly stressed cylinder heads and difficult valving and porting locations which necessitate the use of cooled and lubricated metallic construction.
Various prior art engines have disclosed the advantages of a variable compression ratio in a reciprocating engine and several means for accomplishing this during engine operation are well known. Wallace and Lux (SAE Transactions No. 72 p. 680, 1964), for example, disclose a means of controlling the clearance volume of the cylinder by hydraulically positioning the piston crown above the piston pin. This technique is burdened with the complexity of supplying hydraulic fluid in a controllable manner through rotating and reciprocating members into the most intensely heated and highly stressed region of the engine, namely the piston crown. Another method known in the art is one disclosed by Paul and Humpreys (SAE Transactions No. 6, p. 259, April, 1952) in which the cylinder head of the engine is spring-loaded to allow the clearance volume to change with increased cylinder pressure. This method is mechanically and structurally complex and it also requires intense cooling of the springs in order to prevent premature failure of the mechanism. Still another method of varying the compression in operation applies only to a rocking-beam type opposed piston engine as disclosed by Clark and Skinner (SAE Paper 650516, 1965), wherein a variable compression system was integrated into the Hill engine. This method changes the piston stroke and, thus the total cylinder displacement, by simultaneously altering the rocker ratio between a single transverse crankshaft and the twin connecting rods of the opposed pistons. This technique utilizes a pair of eccentric rocker shafts that are synchronously rotatable within heavily loaded bearings which requires a precise and robust mechanism having critical lubrication problems. In fact, all of the prior art mechanisms described above are vulnerable to intense heat and load exposure.
The history of the internal combustion engine contains an abundance of examples of engines constructed with unusual means for the translation of power (see, for example, Setright, L. J. K., Some Unusual Engines, Mechanical Engineering Publications, Ltd., London, 1975). Whatever the various advantages offered by many of these prior art examples, none overcomes the structural, thermal, mechanical, dynamic and frictional limitations that have been a barrier, heretofore, to the construction of an engine that can operate free of vibration, supplemental cooling and lubrication.